(a) Field of the Invention
The present invention relates to a mobile terminal for efficiently measuring a carrier to noise and interference ratio (CNIR) in an orthogonal frequency division multiplexing (OFDM) mobile communication system and a CNIR measuring method thereof.
(b) Description of the Related Art
Various methods have been proposed for providing diverse services including high-quality and high-speed multimedia services in next-generation mobile communication systems. However, the deterioration in performance caused by multipath fading channels in mobile communication environments has become a serious impediment to realizing such services. Accordingly, an OFDM method has been suggested for efficiently solving the problem of performance deterioration by multipath fading with less complexity.
In the OFDMA method, N subcarriers are divided into groups in a single OFDM symbol such that the subcarriers are not repeated, and more than one of the divided groups (or subchannels) is allocated to each mobile station user.
In addition, information on CNIR may be used for various purposes in the mobile communication system. Channel decoders, such as a turbo decoder, use the information to improve the performance therein. In addition, optimum bit loading may be performed in various channel environments. That is, according to the CNIR or a signal to noise ratio (SNR) measured by the mobile station, the base station transmits user bits with a lower data rate when a channel quality is bad, and transmits those with a higher data rate when the channel quality is good. Therefore optimum system capacity may be obtained.
As described, in order to perform the optimum bit loading according to the channel quality in the base station, the mobile station is required to report a CNIR to the base station after measuring the CNIR in the subchannels allocated to the corresponding mobile station.
Known data symbols should be transmitted to acquire an accurate estimation of CNIR. However, this prior knowledge about the transmitted data can not be available or suffer from system overheads that extra control and/or known messages are required. Specifically, in an IEEE 802.16 OFDMA system, the CNIR of each mobile station is measured for each subchannel since one mobile station uses some subchannels among a frequency bandwidth in one OFDM symbol of a forward link, which may cause a serious load on the system.
A method for measuring the CNIR has been disclosed in U.S. Patent Publication No. 2003-0002471 A1, “Method for estimating carrier-to-noise-plus-interference ratio (CNIR) for OFDM waveforms and the use thereof for diversity antenna branch selection”. In this patent, only a method for measuring the CNIR for diversity selection is disclosed, without disclosing a method for efficiently measuring the CNIR. The above described method is called a training mode.
In addition, there is a method for measuring the CNIR without using a predetermined message or reducing the load on the system. The method is called a blind method, and a decision directed (DD) method is one of the most simple and classic blind methods. That is, the DD method is a method for measuring the CNIR by regarding a determined result as a message predetermined by the base station after determining a demodulated symbol in a determination block. Such a DD method is disclosed in a transaction titled “Accurate Noise Estimates in Multicarrier Systems” in the IEEE VTC 2000, pp. 434 to 438, September 2003, and in this transaction, the method for measuring the CNIR by transmitting a predetermined pilot is compared with the DD method in which the CNIR is measured by regarding a determined symbol as a pilot predetermined by the base station. However, a method for efficiently measuring the CNIR while reducing the load on the system is not disclosed in this transaction, and there is a problem in that the measured CNIR is not accurate due to errors, except in a case of high CNIR.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known in this country to a person or ordinary skill in the art.